Diodes & charging

[rmorris]

I've got a alternator charging issue.
Started with rear not charging, then while troubleshooting the front stopped as well.
1967 T337C, but the VRs upgraded via field approval years ago to the parallel VRs used in later models.
My question is about these diodes (see pic). Got 1 per engine. Looks like it's wired in such a way that 1 wire to battery post on alternator (on top of diode) and other wire going to master solenoid.
My thinking is that the diodes only allows one way traffic for voltage.

When I i just turn on master (engines not yet running), I see 24v on bottom posts for both engines. But on one top post I see negligible (like .8 ), on the other I see the full 24v.
Which reading is correct, what should I see on top and bottom 9f that diode? Also, what readings on the screw terminals on outer diode box?